1. Field of the Invention
The present invention relates to a check valve to be installed in a vacuum sewage pipe providing fluid communication between a source of sewage and a vacuum station of a vacuum sewage collection system, and also relates to a vacuum sewage system having a vacuum sewage pipe provided with such a check valve.
2. Description of the Related Art
FIG. 1 is a schematic view showing an example of a vacuum sewage system. As shown in this drawing, sewage, discharged from houses 100, factories, and the like, flows through gravity-flow-type inlet pipes 101 into vacuum valve units 102 which are buried underground. Each of the vacuum valve units 102 has a vacuum valve 103 operable to be opened when an amount of sewage stored therein reaches a predetermined value. Once the vacuum valve 103 is opened, the sewage is sucked via a vacuum sewage pipe 1, so that the sewage is collected in a sewage collection tank 105 of a vacuum station 104. The collected sewage is then delivered by delivery pumps 106 to a non-illustrated sewage treatment plant or the like.
The vacuum sewage pipe 1 is required to have section valves (gate valves) and inspection ports (test-ball insertion ports) for maintenance thereof A 2002-version vacuum sewage collection system technical manual (published by Japan Institute of Wastewater Engineering Technology) prescribes that the section valves are to be arranged at intervals of not more than 400 m in a main pipe and to be installed at junction points (points A in FIG. 1) where branch pipes (branch pipes 1B in FIG. 1) are joined to main pipes (main pipes 1A in FIG. 1). This manual also prescribes that the inspection ports are to be installed near the section valves and to be arranged at intervals of not more than 200 m.
When the sewage flows from the branch pipe 1B into the main pipe of the vacuum sewage pipe 1, as shown in FIG. 2, the sewage 107 flows not only in a downstream direction (indicated by arrow C), but also in an upstream direction (indicated by arrow D). This is because equal vacuum pressure is developed in all regions of the main pipe including the upstream side of the junction point A. Such a flow toward the upstream side would prevent a smooth flow of the sewage, resulting in an increased loss in the pipes. Thus, as shown in FIG. 3, in the conventional structures, it is recommended to install a check valve 108 near the junction point A so as to prevent the backward flow of the sewage.
Such a check valve can prevent the backward flow and can thus allow the sewage to flow smoothly. However, installation of the check valve itself incurs an increased cost.